1. Field of the Invention
The present invention relates to a heart treatment equipment for treating a heart failure and more particularly to a heart treatment equipment where a heart failure is treated in such a way that right and left ventricles are stimulated at the same time in response to the stimulation of the vagus nerve or the heart rate increase.
2. Description of the Related Art
A cardiac activity is put under the antagonistic control of an automatic nervous system and the automatic nervous system has a sympathetic nerve and a parasympathetic nerve where the parasympathetic nerve of the heart is a vagus nerve and the increase in the sympathetic tone has an excitatory effect on the cardiac activity and the increase in the vagal tone has an inhibitory effect on the cardiac activity.
The inhibitory effect on cardiac activity by the increase of the vagal tone mainly causes a decrease of the heart rate and the arterial blood pressure. The decrease of the heart rate reduces the number of a ventricular premature contraction and the like which induce a ventricular tachycardia and a ventricular fibrillation. The decrease of the arterial blood pressure reduces myocardial oxygen consumption correlatively with the decrease of the heart rate. The decrease myocardial oxygen consumption prevents a myocardial infarction and at the same time prevents the myocardial failure region and its peripheral region from becoming oxygen-deprived. The increase of the vagal tone also suppresses the sympathetic tone directly.
By utilizing such a principle, it is shown a heart treatment equipment in Japanese laid-open patent publication No. 8-38625 (corresponding to U.S. Pat. No. 5,578,061) where the vagus nerve is stimulated and at the same time the sympathetic nerve is stimulated for the purpose of a sympathetic nerve block in response to the detection of the tachyarrhythmia so as to prevent and to treat a tachyarrhythmia.
Further, a heart treatment equipment for stabilizing the cardiac rhythm (heart rate) and for maintaining the cardiac rhythm (heart rate) in the free running cycle of the heart by electrically stimulating the vagus nerve and the sympathetic nerve at the same time was also already proposed (see, for example, a Japanese laid-open PCT patent publication No. 11-514268, which corresponds to U.S. Pat. No. 5,700,282).
The electric stimulation of the vagus nerve has such as an effect of decreasing the cardiac conduction velocity and an effect of decreasing ventricle contraction force caused by a negative inotropic effect other than decreasing the heart rate and the arterial blood pressure. The electric stimulation of the vagus nerve also made distribution of the cardiac conduction velocity non-uniform in the ventricle in connection with the region and the seriousness about the myocardial injury and/or disease of the organic heart disease, so that there was a problem of causing a cardiac conduction disturbance. For example, when a phenomenon where a cardiac conduction system cannot conduct a impulse to the right ventricle or the left ventricle or a phenomenon where a conduction of a impulse is delayed occurs, the efficient blood ejection based on a harmonic contraction of the right ventricle and left ventricle cannot be achieved, so that the heart pumping dysfunction develops. As a result, the cardiac output decreases and an organic heart disease patient whose cardiac function is lowered decreased would have a further deterioration of cardiac function and develops a heart failure. Similarly, the decreasing of a ventricle contraction force caused by a negative inotropic effect would develop a heart failure.
A heart failure means a condition that the heart loses its ability to pump enough blood which is suitable for a metabolism demand and it is caused by various heart diseases and mainly by an ischemic heart disease, a cardiac myopathy, heart valve disease and the like. For many of heart failure patients, a paradoxical movement of a ventricular septum occurs caused by an intra-ventricular conduction disturbance of such as a bundle branch block and an abnormal delay, so that the cooperative contraction of the right and left ventricles is inhibited and the cardiac output is decreased.
In the U.S. Pat. No. 4,928,688 specification, there is shown a heart failure treatment equipment where the right ventricle and the left ventricle are stimulated simultaneously or stimulated with shifted timings in order to recover the cooperative contraction of the right and left ventricles. According to the equipment shown in this patent document, there is shown an equipment where the right and left ventricles are stimulated simultaneously based on heart signals detected from the right ventricle and left ventricle respectively if a ventricular contraction is not detected from either of the right and left ventricles and when the contraction is detected in either one of the ventricles, the heart signal of the ventricular from which the ventricular contraction is not detected will be observed for a short period after that detected time point and that ventricular is stimulated if the ventricular contraction will not be detected.
Further, a method is known where each of mechanical contractions of the right ventricle and the left ventricle and the left ventricle cardiac output are detected by an impedance measurement and the timing of the relative left ventricle stimulation is adjusted for the mechanical contraction of the right ventricle such that the left ventricle cardiac output will become a maximum value (see, for example, the U.S. Pat. No. 6,223,079 specification).
A paradoxical movement of a ventricular septum prevents an effective blood ejection of the ventricle and decreases the stroke volume, so that the heart intends to compensate a necessary cardiac output by an increase of a heart rate such that the heart rate will be maintained high as compared with a normal condition. In case when the pump function of the ventricle cannot follow this increase of a heart rate, the increase of the cardiac output cannot be expected and what is even worse, the cardiac output may decrease oppositely. Therefore, there was a problem that the rapid increase in a heart rate occurs for a heart failure patient even in minimal exertion and he cannot continue exertion such that he feels palpitation, fatigue and the like owing to a fact that the pump function of the ventricle cannot follow this increase of a heart rate.
Further, the stimulation voltage of the left ventricle is generally set at a voltage level over several times as compared with that of the stimulation voltage of the right ventricle. This is because the left ventricle is usually stimulated through a vessel wall of the coronary vein by inserting an electrode lead intravenously to the coronary sinus such that the stimulation threshold itself becomes very high while the right ventricle is stimulated by an electrode which directly contacts the cardiac muscle. Additionally, it is difficult for the left ventricle stimulation to fix the electrode at an aimed region of the coronary sinus and there is a possibility of transferring the electrode after fixed, so that a large margin is set for the stimulation voltage relative to the stimulation threshold in order to surely carry out the stimulation. As a result, the electric power consumption of the left ventricle stimulation increases very much, so that the electric power consumption for the stimulation of both the ventricles where the right ventricle and the left ventricle are stimulated at the same time becomes much bigger. Consequently, there was a problem of shortening the battery life caused by its electric power consumption if the stimulation is always carried out for the left ventricle or for both the ventricles.